A standard double-hung window has a pair of sashes that are vertically slidable next to one another in a rectangular casing formed by vertical side boards and horizontal top and sill boards. The upper sash is guided between an outer stop and a parting strip and the inner sash
As described in U.S. Pat. No. 1,514,513 of Flemming the side boards are spaced inward of the rough frame or masonry opening so as to form two sash pockets or boxes. In addition each of the sashes is suspended from two sash cords which in turn each have an inner end connected to the respective edge of the respective sash and an outer end to a respective weight in the respective sash box. These cords, which may be rope or chain, are reeved over respective pulleys set in the tops of the respective side boards so that as each sash moves up the respective weights move down and vice versa.
The standard upgrade for such an obsolete window is to remove both sashes and fit the casing with a replacement window itself constituted as a rectangular frame containing two new sashes. This replacement window has the necessary channels and balancers for guiding and holding the new sashes.
Thus the replacement procedure entails pulling off the inner stops and parting strips, cutting the sash cords, and removing the old sashes. The sash weights drop to the bottom of the respective sash pockets and are left there. Then the pulleys are removed, as they project into the window opening, the replacement is fitted in place against the outer stop, and the inner stops are replaced to hold the new window in. The replacement window fits the casing tightly and normally engages it with foam insulation so as to form a weathertight fit.
While such a procedure greatly reduces heat loss through the window itself, the pockets remain substantially empty. Even though a dead-air space has some insulation value, the sash pockets themselves are deep enough for convection currents to occur, so that considerable heat loss can occur to each side of the new replacement window at these sash pockets and condensation can cause further problems.